Impurity-defect interaction in polycrystalline silicon for photovoltaic applications. The role of hydrogen

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Impurity-defect interaction in polycrystalline silicon for photovoltaic applications. The role of hydrogen

A. Chari, P. de Mierry, A. Menikh, M. Aucouturier

To cite this version:

A. Chari, P. de Mierry, A. Menikh, M. Aucouturier. Impurity-defect interaction in polycrystalline

silicon for photovoltaic applications. The role of hydrogen. Revue de Physique Appliquée, Société

française de physique / EDP, 1987, 22 (7), pp.655-662. �10.1051/rphysap:01987002207065500�. �jpa-

00245591�

655

Impurity-defect interaction in polycrystalline silicon for photovoltaic applications. The role of hydrogen

A.

Chari,

^{P. de}

Mierry,

A. Menikh and M. Aucouturier

Laboratoire de

Physique

^des

Solides, C.N.R.S., 1, place

^Aristide

Briand,

92195 Meudon Bellevue

Principal Cedex,

^France

(Reçu

le 3 octobre

1986, accepté

^{le 20}

janvier 1987)

Résumé. ²⁰¹⁴ Cet article résume les études effectuées par les ^{auteurs sur} le comportement

physico-chimique

^de

quelques impuretés (P, C, H)

dans le silicium. Les résultats portent ^{sur :} la diffusion et la

ségrégation d’impuretés

dans le silicium mono et

polycristallin,

^la

passivation

des défauts recombinants par

l’hydrogène,

les interactions

hydrogène-dopants.

^{Un accent}

particulier

^{est mis sur le} comportement ^et la diffusion de

l’hydrogène.

Les résultats sont discutés en tenant compte de l’existence de mécanismes

complexes

d’interactions entre

l’hydrogène

^{et les}

impuretés

^ou les défauts.

Abstract. ²⁰¹⁴ An overview of the studies done

by

the authors on the

physicochemical

behaviour of some

impurities (P, C, H)

in silicon is

given.

^{Results concern :} diffusion and

segregation

^of

impurities

^{in mono and}

polycrystalline silicon, passivation

^of

recombining

^defects

by hydrogen, hydrogen-dopant

interaction. A more

focused interest is

given

^on

hydrogen

diffusion and behaviour. The results are

discussed, taking

^{into account}

the existence of

complex

mechanisms of interaction between

hydrogen

^and

impurities

^{or defects.}

Revue

Phys. Appl.

²²

(1987)

^{655-662 JUILLET 1987}

Classification ^.

Physics

^Abstracts

72.80C ^- 61.70N ^- 61.70Y ^- 66.30J

1. Introduction.

The

development

^of

polycrystalline

^{silicon as a}

material for

photovoltaic applications

has raised several fundamental

questions,

^{in relation with tech-}

nical

implications. Technically speaking,

^{the main}

problems arising

^{when this new material was} pro-

posed

^can be summarized as follows :

i)

What will be the consequence of the

polycrystal-

line nature on the

technology

of solar cell fab- rication ? In other

words,

^{would the}

grain-bound-

aries or other defects

(dislocation

arrays,

etc.)

behave for instance as electronic of atomic diffusion short

circuits, modifying

^the

junction profile

^and/or

the behaviour of the cell ?

ii)

^{Will the defects of the}

polycrystalline

^material

hinder the

photovoltaic properties

^{of the}

material,

that is

generation

^{and diffusion of the}

minority carriers, mobility

^{of the}

majority carriers..., leading

also to a

degradation

^{of the}

efficiency

^{of the}

photovoltaic

^{cells ?}

iii)

What would be the

reliability

^{of the solutions}

proposed

^{to cure this above mentioned}

degradation :

specific elaboration,

modification of the process, defect

passivation by hydrogen

^{or other means, ... ?}

iv) Taking

^{into account} all these

factors,

^what

would be the maximum content of different defects

(point,

linear and

two-dimensional)

^{and of different}

impurities

which can be allowed in a « solar

grade »

material to ensure a

possible

^choice of this material for

photocell fabrication ?

These technical

problems

^{lead to several funda-}

mental

questions, concerning

^the

physical

^and

physico-chemical

^behaviour of

polycrystalline silicon ;

^{one can} mention for instance :

- structure of the defects of the material and its influence on electronic and

photoelectric properties,

-

possible grain-boundary

^{fast diffusion and/or}

segregation

^of

dopant

^and

impurities,

- nature of the interactions between

impurities

and

dopants

^{or defects and their} consequence ^on electronic

properties,

- diffusion and

solubility

^{of the}

passivating

^im-

purities (e.g. hydrogen)

^{in this}

material,

-

passivation mechanism,

^{i.e. the nature} of in-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01987002207065500

656

teractions between

the passivant

and the recombina- tion centre.

A

large

number of research work has been done and

published, giving partial

^or definitive answers to some of these

questions [1-4].

This paper

summarizes

some results obtained

by

the authors on

polycrystal-

line and

bicrystalline silicon,

^{which can}

bring

^a

contribution to the

knowledge

^{of the behaviour of}

polycrystalline

^{silicon in the}

following fields :

-

Grain-boundary

^{diffusion and}

segregation

^of

the

impurities (phosphorus,

^{carbon and}

hydrogen)

and their consequences ^{on the} recombination at

grain-boundaries,

^{with a}

special

^{interest on}

hydrogen passivation

^of

bicrystals.

- Introduction and diffusion of

hydrogen

^into

silicon.

- Interactions between

hydrogen

^and

impurities, especially dopants,

^and consequences ^on electronic

properties.

2.

Grain-boundary

diffusion and

ségrégation

^{of im-}

purities.

2.1 PHOSPHORUS. - The

preferential

^{diffusion of}

phosphorus

in silicon

grain-boundaries

may

have,

^if

it

exists, important

consequences ^{on the} diffused

junction profile

^{of the}

photovoltaic

^cells

[5]. System-

atic measurements were conducted on

p-type (B doped,

p ⁼ 1

Ocm) polycrystalline

^{silicon elaborated}

by

the R.A.D. process

[6]

^or

by ingot casting (Wacker Heliotronic).

A radioactivation method was used :

phosphorus

is diffused into the material

by

the conventional borosilicate

glass

process, ^at various

temperatures,

^it

is activated

by

^neutron bombardment and the pro- files are

analysed by grinding

^and

residual activity

measurements

[7] ;

the results are summarized in

figures

^{1 and 2.} Bulk diffusion coefficients of P in Si

are

measured,

^{but no}

grain-boundary preferential

diffusion of P could be observed in this kind of

material,

^{neither from the}

penetration

curves, ^nor from the observation of

autoradiography.

The absence of

grain-boundary

^{diffusion observed}

here is in

disagreement

with results obtained

by

other laboratories

[5, 8]

^on

different

materials. The reason is that R.A.D. and

ingot

^cast

polycrystalline

silicon contain a

large majority

^of

grain-boundaries

in coincidence or near from coincidence.

The measurements of

phosphorus intergranular diffusivity

^mentioned

[8]

^were obtained in

recrystal-

lized silicon with

mostly

^random

grain-boundaries.

2.2 CARBON. - Carbon is often a

majority impurity

of solar

grade polycrystalline

silicon. A

study

^of

carbon diffusion and

segregation

^{has been conducted}

on R.A.D. and Wacker

polycrystalline

silicon. A radioactive tracer method is used : the

specimens

^are

first coated with a

layer

of radioactive

(i5C labelled)

Fig.

^{1. -} Variation of the 32p residual

activity (03B2)

^{as a}

function of the

depth

ⁱⁿ

polycrystalline

silicon. Diffusion anneal : T ⁼

900 °C ;

^{t = 5}

h ;

^No

grain-boundary

^diffu-

sion tail is observed, and the full curve follows the

ILAn - _X = K exp (- X2n/4 Dt)

^law

expected

^{for a pure}

bulk diffusion.

Fig.

^{2. - Arrhenius}

plot Log (D)

⁼

/(l/7r)

^{for bulk}

diffusion of

phosphorus

^into

polycrystalline

^silicon.

7

uC

layer by annealing

^{at moderate}

temperature

)00

°C)

^{in a} sealed silica tube

containing

^the

a14 C03 compound.

Various heat treatments are ten done and the

14C profiles

^are

analysed by inding

^{and residual}

activity

measurements

[7].

^For

[e

segregation studies,

^the

specimens

^{are first}

mogeneized by prolongated

treatment at

high

mperature

in the presence of

Ba 14 C03 (1200 °C, N x ) days)

^{and then annealed at various}

temperatures S

provoke

^the

expected segregation.

Thé segrega-

on is detected

by autoradiography (conventional d

id

high resolution)

^{of the}

specimens [7].

The results on diffusion are summarized in

%ures 3 and 4. Bulk diffusion

coefficients

of carbon to silicon are obtained at different

temperatures

it as for

phosphorus

^no

grain-boundary preferential

ffusion could be detected in this kind of

material,

;ither

by analysis

^{of the}

penetration

^{curves, nor}

by itoradiography.

Furthermore a

strong segregation tendency

^of

rbon to

grain-boundaries

^{has been}

observed.

^Not

Fig.

l the

grain-boundaries

^{are affected}

by

^this segrega- ^diffa

n

phenomenon (Fig. 5).

In order to

try

^to correlate the

segregation ability

.th the recombination behaviour of the

grain- nndaries,

^crossed

experiments

^{are conducted as}

llows

[7, 9].

^A

cartography

^of

minority

^carriers

combination centres is done

by

^E.B.I.C.

(Electron

;am induced

current)

^on

given specimens

^{and thé}

me

specimens

^{are then}

homogeneized

^with

14C

^and

500 micrc

radio of th

250 a

of th

25 50 75 100

^anne

X (CM) x 10-4

Thé

The

3.

^- Variation of the 14C residual

activity (13)

^{as a that}

iction of the

depth

ⁱⁿ

polycrystalline

silicon ; ^{diffusion wher} neal : T ⁼ 1 353

°C ; t

⁼ 2 h 30 min. The

(b)

part ^{of the In}

rve is

perturbed by

^carbide

precipitation phenomena.

^same

le

(a)

part follows the

A ~An ~Xn = (- X2n/4 Dt

^{law ean 1}

give pected

^{for a} pure bulk diffusion. small

10°-

-8,

1. -9

10

1io- 10

il 19 .

6 6,5

7 T-1 x 104 (°k-1)

4. ^- Arrhenius

plot Log (D)

⁼

f(1/T)

^{for bulk}

sion of carbon into

polycrystalline

^silicon.

5. ^-

Autoradiography

^of

14C coupled

^{with EBIC}

e on a

specimen

^of R.A.D. Black dots

(optical )graph)

^or white dot

(S.E.M. micrograph)

^{of the}

graphy

indicate the presence of

14C.

The black lines

.e E.B.I.C.

image

^indicate

recombining

boundaries.

,aled.to obtain the

grain-boundary segregation.

comparison

between the

autoradiography

^of

and the E.B.I.C.

images

^shows

clearly (Fig. 5)

the

recombining

boundaries are also boundaries

^e a

segregation

of carbon is observed.

an electron

microscopy investigation

^{of the}

material, Sharko [10]

has shown that carbon be

precipitated

^{as C-Si}

microparticles

^{in some}

i

grain-boundaries (mostly

boundaries with ^a 1 deviation to

coincidence).

^{Both our and}

Sharko’s results agree ^to prove that carbon seg]

tion or

precipitation

^is

responsible

for the min carrier recombination behaviour of these g boundaries.

2.3 HYDROGEN IN POLYCRYSTALS. - The

hydr

introduction and bulk diffusion

aspect

^{will be} tailed in a

following

^{section 3. The}

problem

^o:

interaction between

hydrogen

^and

grain-bound

is of

great importance

^to understand the mechar of the recombination centre

passivation by

element.

A radiotracer

technique

^{has been} also used in

case : tritium

(3H )

is introduced into

p-type poly

talline silicon

(R.A.D.

^or

Wacker) by

^catl

polarization

^{in acidic salt bath}

electrolyte

^{at If}

labelled with tritiated water

[11]. By photocu:

measurement it is checked that a certain

passiv

of the

recombinating

^{centres is} thus obtained

High

resolution

autoradiographies (0.2

^{03BCm r}

ution)

^are

exposed

^{on the}

specimens

^{either in}

bulk

shape

^{or as thin foils}

ready

for transmit electron

microscopy [13].

The observation of

autoradiography,

^{either in the}

scanning

^ele(

microscope

^{or in the} transmission electron m

scope

(Fig. 6),

^{shows a}

strong interaction

^of

hi

gen with ^some

given grain-boundaries,

^{and 1}

precisely,

^with

linear

^defects

(«

^{extrinsic ç}

cations »)

^{of these}

boundaries.

So the

segreg:

behaviour of

hydrogen

^is

then

very

similar

^{to th} other

impurities.

2.4 HYDROGEN IN BICRYSTALS. ^- In order ^{to t} understand more

completely

the role of g boundaries

impurities interaction,

in the recoml tion and

passivation phenomena,

^a

systematic

was done on

bicrystalline specimens.

^{The matei}

a 03A325 (710) bicrystal

elaborated

by

^Czochi

process. ^{It is} known

[13]

that this

grain-bour

their From

the results

(Fig. 7)

^the

following

^obse

)S10n _{tions can be drawn :} the

Fig.

^{7. - L.B.I.C.}

scanning (electrolytic diode)

^g

silicon

bicrystal (03A3

⁼ 25,

(710))

after different heat t]

ment, before

(2013)

^{and after}

(- - - -) plasma hydro

.m on ation. Incident

light :

GaAs laser

(A

^{= 0.954} 03BCm ; 03B1- in a 50 03BCm ; beam diameter 30

03BCm).

E. M.

itium.

indi-

The

preceeding

^results

[13]

^{on the role of}

thermal treatment are confirmed. The boundai

slightly recombining (recombination

^{rate s =}

103

cm.

s-1)

^{after 450 °C and}

strongly

^recombi]

try

^to

(s

^{= 2.5 x}

104

^cm.

s-1)

^{after 750 °C and 900 °C}

;rain- nealing.

^The

hydrogen

^{treatment leads to a} comf bina-

passivation

^{of the 450 °C annealed}

boundary

^but

study

^{no influence on} the behaviour of the 750 °C rial is 900 °C annealed

specimens.

^The

penetration

ralski

hydrogen

^{has been checked}

by

^S.I.M.S.

(seconc

idary

^{ion mass}

spectroscopy) profiling

after deuter

659

plasma annealing.

This very ^recent result of a

study

still under continuation _proves that :

i)

^the

impurity segregation responsible

for the creation of recombin-

ing

^{centres is not}

unique,

^{as the 450 °C}

annealing has

not the same effect as the 750 °C or 900 °C anneal-

ing ; ii) hydrogen

^{does not}

passivate

all the recombi- nation centres and interacts

preferentially

^{with the}

centres created at 450 °C. An

autoradiography study

after tritium introduction is to be done on those

bicrystals.

3. Introduction and diffusion of

hydrogen

^into

silicon.

Hydrogen

^can be introduced into silicon

by

^several

means : bombardment in a Kaufman source

[14, 15], plasma annealing [16], electrolyte charging [12].

Plasma

annealing

^and

electrolyte charging

^{have been}

investigated

^{in the}

present study,

^{in order to}

quantify

the diffusion mechanisms of

hydrogen.

^{For this}

study, monocrystalline silicon

^{is used as a first}

step.

3.1 HIGH TEMPERATURE INTRODUCTION BY PLAS- MA ANNEALING. - The diffusion

profiles

^of

hydro-

gen into silicon ^at

high temperature

^are then studied

by

^S.I.M.S. of deuterium

(hydrogen

^is

replaced by

deuterium in the

plasma atmosphere (1 mbar)).

^Two

different

plasma

sources were used : a R.F.

plasma (13.56 MHz,

²⁰

W, specimen

^{inside the}

glow

^dis-

charge)

^{and a microwave}

plasma (2

⁴⁵⁰

MHz,

⁶⁰

W, specimen

outside the

glow discharge).

Some results are

given

ⁱⁿ

figures

^{8 and 9 at various}

temperatures

^{and for} various materials :

p-type (6

^x

1016

^{at. B .}

cm-3), n-type (1018

^{at. P.}

cm-3)

^and

undoped.

From this non-achieved

investigation,

some

provisory

conclusions may be drawn :

a)

^the

solubility

^of

hydrogen

in silicon is in all case, « reverse », that

is, decreasing

^{when the tem-}

perature

is raised.

b)

^the

apparent solubility

^for

given

^conditions

(i.e.

^{for a}

given plasma source),

^is

strongly depen-

dent on the

doping type

^and

doping

level. It is one

order of

magnitude larger

ⁱⁿ

p-type

silicon than in n-

type

^or

undoped

^silicon.

c)

the diffusion

profiles

^{cannot be}

analysed by

^a

simple

^diffusion

mechanism ;

each of them is the result of the addition

of,

^at

least,

^{two diffusion}

profiles.

The observation of such

complex

^diffusion

profiles

^{has been}

already

mentioned in studies on p-

type

^silicon

[17, 18],

^{and the}

explanation usually given

is the interaction

phenomena

^between

hydro-

gen and boron

(see following

^Sect.

4).

d)

the maximum values of the diffusion coeffi- cients which can be deduced from the

deepest part

^of the

profiles

^{are of the order of}

10-12 cm2. s-1

at

150 °C and

10- Il cm2. s-1

at

320 °C,

ⁱⁿ

agreement

with the literature

[19].

^{It must be mentioned that}

Fig.

^{8. -} Deuterium S.I.M.S.

profils

ⁱⁿ p-type

(6 1016 at.B.cm-3)

in silicon after various deuterium

plasma annealings. a)

^R.F.

plasma ; (1)

^{T =}

150 °C,

t ⁼ 3

h ; (2)

^{T = 260}

°C, t

^{= 3 h.}

b) microwave plasma ; (1)

^{T =}

150 °C ; t = 3 h ; (2) T = 320 °C ; t = 3 h.

these values concern

hydrogen

detectable for the

given

conditions of S.I.M.S.

analysis (detection

^limit

1015

to 5 x

1015 at . cm- 3 of deuterium).

3.2 ELECTROCHEMICAL PERMEATION OF HYDRO- GEN. - The electrochemical

permeation technique

has been used to detect very small fluxes of

hydrogen diffusing through

^a silicon membrane at room tem-

perature [20].

^{In this kind of}

experiments,

^{a thin}

(~ 100 03BCm)

^{membrane of}

monocrystalline

^silicon

(p-

type

^or

n-type

^{with a n+}

/p junction

on ^one

surface)

is covered on both sides

by

^a

palladium layer.

^The

« entry

^{face » is}

exposed

^{to an} acidic solution

(H2S04,

¹

N)

^and

cathodically polarized

^to

produce

atomic

hydrogen (constant

^{current 10}

mA/cm2).

^The

« down stream face » is

exposed

^{also to an elec-}

trochemical cell

anodically polarized (V

⁼

100

mV/ECS)

ⁱⁿ

H2S04,

^{1 N.} The anodic current

i A

^{measured in the « down stream » cell is a measure}

of the flux of

hydrogen,

reoxidized in this

cell,

^which

have crossed the membrane. The

palladium coating

ensures the current

transport.

^The

permeation

^curve

i A

⁼

f(t) gives

the variation of the

hydrogen

^{flux as}

660

Fig.

^{9. -} Deuterium S.I.M.S.

profiles

in silicon after deuterium microwave

plasma annealing ; a)

ⁿ type

(1018 at.P.cm-3) ; (1) T = 150°C ;

t=3h;

(2)

^T=

320 °C ; t

^{= 3} h ;

b) undoped

silicon ;

(1)

^T

= 150 °C ; t = 3 h ; (2) T = 320 ° C ; t = 3 h.

a function of the time

(Fig. 10).

^The mathematical deconvolution of this curve

[21]

^{leads to the}

apparent

.diffusion coefficient of

hydrogen

^{at room}

tempera-

ture. The

thermodynamical

conditions are such that the

thermodynamic fugacity

^of

hydrogen

^{at the}

«

entry

^{face » is} very

high (larger

^than

10 000

atm.) [21].

^As often in this kind of exper-

iments,

^the mathematical

analysis

^{of the}

permeation

curve shows that the Fick’s law for diffusion is not

satisfied. Such effect is

usually

^a consequence of

complex

^diffusion

phenomena,

^{more or} less hindered

by trapping

^of

hydrogen by

the surface or defects in the material

[21].

^{In the}

present

^{case, it was}

possible

to show that the

permeation

^{curve of}

figure

^{10 can}

be described

by

^{two sets of}

apparent

^diffusion coefficients. One set of values

(« strongly-trapped » hydrogen)

^are of the order of

10-10 cm2. s-1,

^the

other

^set

(«

diffusible ^»

hydrogen)

is of the order of

10-9 cm2.s-1 [22].

Both values are orders of mag- nitude

larger

^than

expected

^{from the}

extrapolation

to room

temperature

of the coefficients obtained at

high temperature [19].

This result means

again

^that

Fig.

^{10. -}

Hydrogen permeation

^flux normalised to

steady

^state

permeation

flux,

through

¹⁰⁰ )JLm thick mem-

brane of p-type ^{silicon as a function} of time. Electrochemi- cal introduction and detection of

hydrogen

^{at room} temperature.

hydrogen

diffusion is a very

complex phenomenon.

Under cathodic

charging, i. e. ,

under very

high

^",

fugacities

^{of atomic}

hydrogen,

^{small amounts of this}

element can diffuse at rates much

higher

^{than the}

species

^detected

by

^S.I.M.S.

analysis

^{of the}

plasma

treated

specimens.

4.

Hydrogen dopant

interaction.

Hydrogen

introduced in

p-type

silicon has another

important

effect : for

large quantities

^of

hydrogen,

one observes a neutralization of the

dopant, leading

to a

spectacular

increase of the near-surface resistivi-

ty

^{and a}

strong

decrease of the

majority

^carrier

concentration

[16-18].

This effect has been observed and measured in

p-type

^{silicon submitted to the}

hydrogen plasma. Profiling

^{of the}

majority

^carrier

concentration under the surface is obtained

by

measurement of the

voltage dependence

^{of the}

capacity

^{of a} diode between mercury and the

specimens.

To extend the

profile

^at

large

^concen-

tration,

successive

etchings

^of the surface are done.

The results

(Fig. 11)

confirm thé

strong

neutraliza- tion effect of

hydrogen

^{on the}

dopant,

^{in the}

regions

where

large quantities

^of

hydrogen

have diffused.

The neutralization

depth

^{is in}

good agreement

with

the

concentration

profiles

of deuterium measured

by

^S.I.M.S.

The

physical

^{mechanism of the}

dopant

neutraliza- tion is still under discussion

[23]

^but it is clear

that,

from the

physicochemical point

^of

view,

^such

strong interaction

will

provoke important

modifications of the diffusion

mechanisms.

We are here

typically

ⁱⁿ

the case of

strong trapping phenomena.

5. Général discussion and conclusion.

The

problem

^of

impurity-impurity

^and

impurity-

defects interactions in silicon and their consequences

on electronic and

photoelectrical properties

^of

poly-

661

Fig.

^{11. -}

Acceptor profiles

from the surface in p-type silicon

(6

^x

1016 at.B.cm-3)

^after

hydrogenation plasma annealing (320 °C,

⁶ h ;

150 °C,

³

h).

crystalline

silicon is far to be

completely solved.

^The

results

given

here indicate some

interesting

^{trends :}

a)

No noticeable

grain-boundary

^{diffusion of the}

dopant

^{exists in the}

strongly

^textured

large grain

materials

developed

^for

photovoltaic applications.

b)

^The

problem of impurity segregation,

^also

dependent

^{on the}

grain-boundary

structure, ^{is of}

major importance

^to understand the recombination behaviour of the boundaries

and, probably,

^other

defects

(dislocations) [23].

c)

^The

hydrogen passivation

^{of the} recombination

centres cannot be

only

described in terms of structur- al defect interaction with

hydrogen,

^{as often done}

(e.g. dangling

^bond saturation

by hydrogen) ;

^the

microchemical

aspect

^{has to} be taken into account,

as recombination centres of chemical

origin

^{are also}

passivated by hydrogen.

d)

^This

problem of hydrogen passivation

^{is be-}

coming

^{even more}

complex

^{if one takes into account}

the

hydrogen dopant

interaction. The fact that

hydrogen

^{is able to} neutralize with a very

high efficiency

^the

acceptors

^{must have an influence on}

the passivation

mechanisms.

e) Hydrogen

diffusion into silicon is not a

simple

mechanism. At least two or three

species

^of

hydro-

gen exist in the material with

respectively decreasing

solubilities and

increasing

diffusivities.

Outgasing (exodiffusion) experiments

^{as a function of}

tempera-

ture, ^and

high temperature permeation experiments

are necessary ^to

separate correctly

the behaviour of these

species.

^{At the}

present state,

^the

hydrogen- dopant

interaction seems to be the

predominant

factor

influencing

the diffusion

phenomena.

Acknowledgments.

This

study

^was

partly

^financed

by

^COMES

(Commis-

sariat à

l’Energie Solaire),

^AFME

(Agence Française

pour la Maîtrise de

l’Energie)

^{and PIRSEM}

(Projet interdisciplinaire

de Recherche du CNRS sur l’Ener-

gie

^{et les Matières}

Premières).

The authors are

grateful

^{to J.} Chevallier

(CNRS, Meudon)

and N. Proust

(Thomson, Orsay)

^{for the}

access to

hydrogen

^{and deuterium}

plasma equip-

ments.

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